In orthopedics it is known to use various types of reconstruction assemblies to repair bone joints that have become deteriorated, damaged or degenerative, such as due to trauma or disease. Some reconstructions involve the use of various components such as bone screws, plates, bone grafts, fusion implants and other components. Depending on the type and method of reconstruction selected, complete stabilization with no movement may be selected, or a predetermined amount of controlled movement may be selected. In one technique of spinal reconstruction, for example, fusion of adjacent vertebrae is achieved using one or more plates fastened to adjacent vertebral segments in order to join the vertebral segments in a predetermined relationship for stabilization, sometimes installing a fusion device such as an implant or bone graft.
While complete fusion and, thus, resultant loss of movement between adjacent vertebrae is sometimes prescribed, fusion does limit movement and in the long term may adversely affect vertebrae adjacent to the fused joint by imposing heightened stress and wear. An alternative to fusion using motion preservation devices restores significant motion and disc space height which minimizes stress concentrations and pain.
The various known systems for allowing controlled movement of joint reconstructions have shortcomings. Such shortcomings include lack of versatility so as to require multiple configurations and sizes of hardware on hand during surgery; prohibitively complex or expensive components; lack of anatomical correspondence with resultant poor fit, high stress concentrations and unnatural load forces on adjacent or fused bone segments; and other shortcomings. Known motion preservation devices are generally restricted to only very stable constructs and degenerative disc disease cases, which is only 5%-10% of all cases.
One known device involves a first, or upper, component; a second, or lower, component; and a middle component. The upper and lower components are generally similar and each includes a base portion adapted to be fixed to an end-face of one of two adjacent vertebrae. Each of the upper and lower components further includes a cup portion formed by an axially extending annular wall having generally concentrically contained therein a generally concave surface. The middle component comprises upper and lower portions that are convex and, preferably, a middle circumferential flange. The upper and lower convex portions are adapted to be matingly received in the respective concave portions when the middle component is positioned between the upper and lower components. The middle component, thus, can slide relative to the upper and lower components due to the relative movement of the convex portions against the concave portions. The middle flange limits movement when it engages the respective annular walls. Varying the shape and/or size of the flange will resultantly vary the amount of allowable pivot. The movement described allows adjacent vertebrae to move relative to each other in a generally pivotal manner at two points of movement. If desired, the annular wall of one or both of the cup portions may be of varying height along its circumference, so the when the system is fully assembled, the attached vertebrae are held in a relatively angled relationship intended to reproduce naturally occurring lordotic orientations.